Hydrogen sorption properties of the composite system 2NaBH

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Hydrogen sorption properties of the composite system 2NaBH ₄ +MgH ₂

Vom Promotionsausschuss der Technischen Universität Hamburg-Harburg

zur Erlangung des akademischen Grades Doktor der Naturwissenschaften (Dr. rer. nat.)

genehmigte Dissertation

von Claudio Pistidda

aus Sassari, Italien

2011

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Vorsitzender des Prüfungsausschusses : Prof. Dr. J. Weißmüller 1. Gutachter: Prof. Dr. R. Bormann 2. Gutachter: Prof. Dr. T. Klassen Tag der mündlichen Prüfung: 9.12.2011

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Abstract of the PhD Thesis

Hydrogen sorption properties of the composite system 2NaBH

4

+MgH

2

Claudio Pistidda

The hydrogen storage properties of the system 2NaBH4+MgH2 are investigated as a model system for the class of Reactive Hydride Composites based on MgB2. The hydrogen absorption and desorption mechanisms are thoroughly elucidated. The effect of the applied hydrogen pressure and temperature on the hydrogen absorption mechanism is studied. For a given set of hydrogen pressure and temperature the dependence of the absorption reaction mechanism on the ratio of the starting reactants is also investigated.

For the desorption reaction, considerable kinetic improvements are achieved by heating the as milled material in hydrogen pressure. The possibility to improve the desorption reaction by an innovative method of enhancing the effective contact area between reactants is also proposed. This method opens a new path for the kinetic enhancement of multi-compound reaction in the solid state.

Abstract der Dissertation

Wasserstoffspeichereigenschaften des Kompositen-Systems 2NaBH

4

+MgH

2

Claudio Pistidda

In dieser Arbeit wurden die Wasserstoffspeichereigenschaften von 2 NaBH4 + MgH2 als Modellsystem für die so genannten Reaktiven Hydrid Komposite untersucht. Der allgemeine Reaktionsmechanismus der Wasserstoffabsorption und -desorption wird ausführlich erläutert. Zudem wird der Einfluss von Temperatur und Wasserstoffdruck auf den Mechanismus der Absorption untersucht. Für bestimmte Temperaturen und Wasserstoffdrücke konnte eine Abhängigkeit des Mechanismus vom Verhältnis der Edukte gezeigt werden. Die Kinetik des Desorptionsprozess konnte signifikant verbessert werden, indem das Material nach dem Mahlvorgang unter Wasserstoffatmosphäre erhitzt wurde. Weitere Verbesserungen der Desorption konnten durch ein innovatives Verfahren zur Steigerung der effektiven Kontaktfläche zwischen den Edukten erreicht werden. Dieses Verfahren ermöglicht einen neuen Weg zur Verbesserung der Kinetik von Multi-Komponenten-Festphase Reaktionen.

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1 Introduction

Since the late 19^th century, humankind has known an uninterrupted period of industrial and economic growth. Discovery of fossil fuels, which provide easy and rapid access to energy, has been the main factor making this possible. However, continuation of this trend of growth is becoming increasingly problematic. According to recent studies ^{[1, 2]} in the next few decades the fossil fuels, on which almost the total world energy supply relies, will certainly reach a peak (figure 1.1). After that point and without economically suitable alternatives, the economy will suffer an inevitable crisis. From the environmental point of view, excessive use of fossil fuels has disturbed the global climate equilibrium, due to the damaging greenhouse gas emissions ^{[3, 4]}. In addition, oil recovery from bituminous sands or out of the deep sea carries high environmental risks as clearly shown by the oil spill disaster caused by the Deep water Horizon drilling rig explosion in April 2010.

As an alternative to fossil fuels, hydrogen is widely regarded as a key element for a potential energy solution, capable of solving the issues of both environmental emissions and energy sustainability. Differently from fossil fuels such oil, gas and coal, hydrogen is not a primary energy source, but rather a secondary energy source since it must be produced itself using energy. However, the possibility to produce hydrogen utilizing several and different intermittent renewable resources such as solar energy, wind energy, etc. shows several advantages. On the one hand it will contribute to a drastic reduction of pollutants released in the air, and on the other hand it will significantly contribute to the security of energy supply.

In addition, the implementation of the hydrogen as “energy carrier” would result in an effective and synergic utilization of the renewable energy resources. A major obstacle for the use of hydrogen as energy vector is represented by its utilization in the transportation sector.

In fact, although for a given amount of energy, hydrogen has the highest energy density by weight if compared to any common fuel, it needs a storing volume which is several times bigger than that requested by common fossil fuels.

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Figure 1.1: Production of: natural gas liquids extraction (NGL), polar crude oil extraction (Polar), offshore crude oil extraction (Deepwater), heavy crude oil extraction (Heavy etc.);Oil extraction in: China (China), in the gulf of Mexico (ME.Gulf), in south America (L.America), in the Asia and Pacific ocean area (Asia-Pacific), in Africa (Africa), in Europe (Europe),in Russia (Russia), in the contiguous United States (US-48), in countries different from the above mentioned (Other).

In this respect, hydrogen storage technology is considered a key roadblock towards the use of H2 as an effective energy carrier for vehicular applications. So far three main options for storing hydrogen exist: highly pressurized gas, liquefied hydrogen and in metal hydrides chemically bonded hydrogen. The most common method to store hydrogen is to compress it to high pressure gas cylinders. However, pressures in the range of more than 700 bar are required in order to achieve reasonable volumetric storage capacities. Liquid hydrogen offers a gravimetric density higher than compressed gas at ambient temperature. In fact 1 kg of hydrogen gas at atmospheric pressure (1 bar) and ambient temperature (295 K) occupies 11.125 m³, whereas in case of liquid hydrogen it is possible to store 754.275 kg of hydrogen in the same volume at ambient pressure. Unfortunately, the condensation temperature of hydrogen at 1 bar is 20 K only and cryogenic vessels require very efficient thermal insulation

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in order to keep the hydrogen liquid. Moreover, despite good insulation, a daily boil–off rate of not less than 1% is inevitable if no additional energy is used to cool the system. On the other hand it is well known that several metals, and in particular transitions metals, have a high affinity for the hydrogen. This high affinity leads to the reaction between hydrogen (H2) and the metal (M) with consequent formation of a metal hydride (MHx) phase. This process can be described as follows:

(1) M + (x/2)H2↔ MHx

Depending on the reacting metal, different types of metal hydrides can be formed. In case of metals like Pd ^[5], V, Nb and U, in the hydrogenation process the hydrogen atoms occupy only interstitial sites without causing the topological structure of the metal to change. These hydrides are called interstitial hydrides. In metals such Mg, Pd, Zr and Ti the absorption of hydrogen leads to the formation of crystalline hydride phases ^[6]. Several hydrides like those of Mg ^[7-11] and Pd ^[12-15] have been studied for decades as possible hydrogen storage materials. However, for vehicular applications the gravimetric capacity of these systems is generally too low. In addition, in those cases with higher gravimetric storage capacity the heat of reaction i.e. the value of reaction enthalpy of the hydrogenation reaction is either too strong or too weak for practical applications. Two examples are the hydrides AlH3 and MgH2. In the first case the value of reaction enthalpy appears to be too small, in the latter too high. Alane (AlH3) has a gravimetric hydrogen capacity of ~10 wt.%, however, due to the weak hydrogen binding energy (enthalpy desorption = 5-8 kJ/mol H2), its direct formation from Al and H2 is impossible. Magnesium hydride possesses a high hydrogen gravimetric capacity of about 8 wt.% as well, however, in contrast to AlH3 its strong binding energy (enthalpy desorption = 75 kJ/mol H2) leads to an equilibrium pressure of only 1 bar at 300 °C. Therefore the tailoring of the reaction enthalpies is a key issue for developing suitable metal hydrides for hydrogen storage. A widely used approach to tune the thermodynamic properties of conventional metal hydrides is the alloying with others elements. However, as main drawback the alloying causes a drastic reduction of the hydrogen storage capacity.

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1.1 Complex hydrides for hydrogen storage

Recently, because of their high hydrogen storage capacity, “complex hydrides”

attracted considerable attention as potential hydrogen storage materials. This class of material consists of hydride species composed by metal cations (e.g. often lightweight alkali or alkaline heart such Li, Na, Mg or Ca cations) and hydrogen-containing “complex” anions such as alanates ^[16], amides ^[17] and borohydrides ^[18]. Although this class of hydrides has been known for a long time (the first report on pure metal borohydride was published in the beginning of the last century ^{[19, 20]}) they were initially not considered suitable for reversible hydrogen storage purposes. This lack of initial interest can be traced to their apparent irreversibility. In fact, the products of their thermally activated hydrogen desorption could not be rehydrogenated unless very harsh condition were applied ^{[21, 22]}. In addition, though the possibility to employ them as “one pass” hydrogen-storage system was widely investigated, the on-board irreversibility of the rehydrogenation process made these materials not suitable for automotive applications.

In the 1996 Bogdanovic and Schwickardi ^[16] were the first to demonstrate the concrete possibility of reversibly store hydrogen in titanium-based doped NaAlH4 at moderate temperature and pressure conditions. Since then, many efforts have been made to optimize complex hydrides as potential hydrogen storage materials for vehicular applications.

However, before complex hydrides can be efficiently employed in the transportation sector, the issues connected with their high thermodynamic stability and sluggish sorption kinetics still have to be addressed.

In 1967 Reilly et al. discovered the possibility to change the reaction enthalpy of hydride containing composites by mixing them with additives which react reversibly with the hydride during desorption to form a stable compound. He could show that at the expense of hydrogen capacity the reaction enthalpy of a 3MgH2+MgCu2 composite is lowered if compared to pure MgH2[23]. Recently, this approach of Reilly et al. has been modified by the Chen et al. ^[24] and later Vajo et al. ^[25] and Barkhordarian et al. ^[26] by using mutually destabilizing hydride mixtures which are also called Reactive Hydride Composites. This concept offers the advantage that the high gravimetric storage capacity of the single hydrides is maintained. Very interesting examples for such hydride mixtures are 2NaBH4 + MgH2, 2LiBH4+MgH2 and Ca(BH4)2+MgH2. One clear illustration of the advantages of using a

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hydride mixture instead of a single complex hydride, can be given using as prototype systems NaBH4 and 2NaBH4+MgH2 (figure 1.2). Pure NaBH4 has a gravimetric hydrogen capacity of roughly 8 wt.% and dehydrogenates according the following reaction:

(1) NaBH4→NaH + B + 3/2H2

The enthalpy change for reaction 1 is equal to 90 KJ/mol H2, resulting in an equilibrium pressure of 1 bar at ~500 °C ^[27]. Instead, the dehydrogenation of the corresponding composite system 2NaBH4+MgH2 proceeds as follows:

(2) 2NaBH4 + MgH2 ↔ 2NaH + MgB2 + 4H2

In this case the gravimetric hydrogen storage capacity is equal to 7.8 wt.% but the reaction enthalpy is only 62 kJ/mol H2, which results in an estimated equilibrium pressure of 1 bar at 350 °C. The formation of MgB2 alongside with the decomposition of NaBH4 results in a significant decrease of the overall desorption reaction enthalpy. The enthalpy decrement is mirrored in a lowering of the estimated equilibrium temperature by roughly 150 °C. In addition, the reduced amount of heat which has to be led away from or led in of a potential tank system strongly contributes to improve the energy efficiency of the system.

Figure 1.2: Enthalpy diagram for the systems NaBH4 and 2NaBH4+MgH2.

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Although, alloying and the use of selected hydride mixture are powerful tools for tuning the thermodynamic properties of a certain hydrogen storage system, the issue of sluggish kinetics is perhaps the primary challenge associated with this class of materials.

Hydrogenation and dehydrogenation reactions in complex hydrides and mixtures of complex hydrides are generally more complicate than in conventional metal hydrides. This difference is mostly due to the involvements of the constituent elements in various reaction steps. For example, the hydrogen desorption from a complex hydride such as a borohydride, requires the bond-breaking of anionic complexes ([BH4]^-) and recombination of the H atoms to form H2

molecules. In addition, the constituent metal elements may have to undergo long range diffusion before the next reaction step can occur or before a final product can be formed.

In the last decades several catalysts or additives have been investigated and employed in order to enhance the sorption properties of complex hydrides. Significant kinetic improvements were reported, however only in a few cases it was possible to clearly identify the atomistic mechanism responsible for the enhancement. As in case of conventional metal hydrides for the complex hydride also the particle-crystallite size distribution of the reactants plays a crucial role on determining the sorption properties of the system. Although, ball milling is a well-known technique and is widely employed on downsizing materials for hydrogen storage, the advantages gained by the use of such method in certain cases vanish upon the material cycling. A method recently applied in order to ensure a fine and stable particle size distribution, is the confinement of the hydride materials, in high surface area scaffolds.

Examples of hydrides to which this method was successfully applied are LiBH4 [28] and NaAlH4 [29]. However, although this approach effectively improves the hydrogen sorption properties of the hydride material it drastically reduces the overall hydrogen capacity of the system ^[30].

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1.2 Scope of the work

In the present work, the hydrogen sorption properties of the Reactive Hydride Composite system (RHC) NaBH4/MgH2 are investigated in detail. As pointed out above, the system 2NaBH4-MgH2 has a theoretical gravimetric hydrogen capacity of 7.8 wt.%, and an overall reaction enthalpy of 62 kJmol^-1H2 which results in an estimated equilibrium pressure of 1 bar at 350 °C ^[31]. Although for this system the reaction enthalpy value is higher than the required for onboard applications, its study is of primary importance for achieving a basic understanding of the sorption properties of Reactive Hydride Composites. In order to achieve such knowledge both the absorption and desorption were characterized stepwise by means of volumetric analysis, X-ray diffraction, calorimetric technique and solid state nuclear magnetic resonance method.

As stated before, in the Reactive Hydride Composite systems the absorption/desorption processes are characterized by the involvement of the constituent elements in various reaction steps. The reaction steps in which hydrogen is involved, are characterized by specific equilibrium conditions (temperature and pressure). Therefore, since hydrogen takes part in the most of the reaction steps, it is likely that the hydrogen pressure influences the course of the absorption/desorption processes. Clear evidences of the applied hydrogen pressure effect on the sorption reaction mechanism of the Reactive Hydride Composites were recently reported by Vajo et al. ^[32], Bosenberg et al. ^[33] and Price et al. ^[34] regarding the desorption properties of the system 2LiBH4+MgH2 2LiH+MgB2.

One of the aims of this work is to understand the impact of the applied hydrogen pressure on the pathway and reaction products of 2NaH+MgB2 absorption process. In order to address this task, the first part of this work focuses on the absorption reaction performed under the pressures of 50, 25 and 5 bar H2. Moreover, for the hydrogen absorption performed under 50 bar H2, the effect of the NaH/MgB2 molar ratio on reaction pathway and reaction products was studied. In particular the following compositions were investigated:

1.5Na/MgB2, NaH/MgB2 and 0.5NaH/MgB2.

As pointed out above a problem associated with the use complex hydrides and mixtures of complex hydrides is their high kinetic stability. In this work, the possibility to enhance the 2NaBH4+MgH2 hydrogen desorption reaction, by surface modifications is proposed. In a first step, the observed kinetic enhancement subsequent to a combined heat and

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hydrogen treatment of the as milled 2NaBH4+MgH2 is reported and discussed. Then, the discovery of a new approach which allows achieving fast desorption reaction improving the contact area between reactants in the mixed Reactive Hydride Composite system 2NaBH4+MgH2 is also proposed.

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2 Experimental section

In this section a brief description of the materials, techniques and utilized instrumentations is given. Due to the multiplicity of the experimental conditions the detailed experimental parameters will be reported later in the corresponding sections.

2.1 Materials

Hereunder the materials used throughout the present work are reported. They are described in terms of purity, and provenance (Table 2.1):

Material Purity Company

NaBH4 98% Alfa-Aesar

NaH 95% Sigma-Aldrich

MgH2 95% Goldschmidt

MgB2 99.999% Alfa-Aesar

Table 2.1: Materials description.

The powder mixture 2NaBH4+MgH2 was prepared using NaBH4 (98% purity) and MgH2

(95% purity) purchased from Alfa-Aesar and Goldschmidt, respectively.

The powder mixture 2NaH+MgB2 was prepared using NaH (95% purity) and MgB2 (99.99%

purity) purchased from Sigma-Aldrich and Alfa Aesar, respectively.

During the compilation of this work we observed a sensible variation of the 2NaBH4+MgH2

sorption properties for different NaBH4 batches.

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2.2 Sample preparation

All the investigated materials were prepared at the Helmholtz-Zentrum Geesthacht by high-energy ball milling using a Spex 8000 shaker mill or a Fritsch P5 mill. In both cases, hardened stainless steel vials and balls were used. The 2NaBH4+MgH2 samples were prepared by first pre-milling MgH2 for 20 hours in a Fritsch P5 planetary ball mill, with a ball to powder ratio of 10:1. Then NaBH4 was added to the pre-milled MgH2 and the mixture milled for additional 20 hours in the planetary ball mill with a ball to powder ratio of 10:1. The ball milled (2NaBH4+MgH2) mixture was divided in three batches. A first batch of material was directly employed for the desorption studies of the as milled system. The second portion of the material was heated in a Sievert´s type apparatus up to 300 °C and then kept one hour under isothermal conditions at 50 bar of hydrogen pressure. For this sample also the desorption properties were investigated (section 3.4.2.1). The last batch of material was furtherly divided into two parts to study the effect of exposure to moistened atmosphere on the desorption properties of the 2NaBH4+MgH2 mixture. For this purpose, the milled 2NaBH4

+ MgH2 (500 mg) was charged in a 100 ml double neck round-bottom flask, which was then evacuated. Subsequently, 2 ml of distilled water were poured in a 100 ml single neck round- bottom flask. The two flasks were connected by means of a 10 cm rubber tube. At this point, the evacuated vessel containing the powder was open leaving the total system at a pressure lower than the atmospheric pressure (the flask containing the powder and the flask containing the water). A first batch of material was prepared exposing the milled powder for one hour to the water moisture, and a second one increasing the exposure time to two hours. The exposed materials were then dried under dynamic vacuum (10^-2 bar) for 12 hours at room temperature.

The xNaH+MgB2 samples were prepared charging simultaneously NaH and MgB2

into a hardened steel vial and subsequently milling them for 1 hour in a Spex 8000 ball mill, with a ball to powder ratio of 10:1. Handling and milling was always performed in a dedicated glove box under a continuously purified argon atmosphere.

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2.3 Kinetic characterization

To evaluate the hydrogen sorption properties of the investigated materials two Sievert´s apparatus designed by HERA Hydrogen System and Advanced Materials Corporation were used respectively at the Helmholtz-Zentrum Geesthacht and at the JRC’s Institute for Energy in Petten (Netherland). The measurements performed at JRC´s were carried out by Dr. Francesco Dolci. The amount of material utilized for each measurement is about 100 mg for the Hera volumetric apparatus and approximately 600 mg for the Advanced Materials Corporation apparatus. The hydrogen absorption measurements were carried out in a range of applied pressures between 5 and 50 bar, by heating up the samples from room temperature (heating rate of 3 °C/min) to the desired final temperatures and then keeping it under isothermal conditions until the end of the experiment. The hydrogen desorption measurements were performed under static vacuum conditions (starting pressure 10^-2 mbar) heating up the sample from room temperature to 450 °C (heating rate of 3 °C/min) which was then then kept under isothermal conditions. The hydrogen release and uptake was determined by measuring the differential pressure between sample holder and an empty reference. Sample holder and reference are of identical design and are subjected to identical pressure and temperature conditions. The measured change in differential pressure is a measure of the amount of released or absorbed hydrogen.

2.4 Thermal analysis

Thermodynamic investigations during hydrogen absorption on the system 2NaH+MgB2 were performed by HPDSC Netzsch DSC 204 HP Phoenixat the Karlsruhe Institute of Technology. The HPDSC measurements were carried out in dynamic mode under constant pressure in a range of temperature of 25-400 °C, with a constant heating rate of 5

°C/min. The amount of material used for each measurement was roughly 20 mg. The crucibles and lids used as sample holder and reference are made of Al2O3 and were directly provided by Netzsch. The measured The HP-DSC apparatus was placed in a dedicated glove box under a continuously purified argon atmosphere.

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2.5 Ex situ X-ray diffraction

X-ray diffraction is a powerful technique to gain a wide range of structural information regarding the investigated materials. In fact quantitative and qualitative analysis of the crystallite phase compositions, crystallographic structures and crystallite domain sizes are possible by means of a normal laboratory X-ray diffractometer. In this work X-ray diffraction analysis were carried out using a Siemens D5000 X-ray diffractometer, a Philips XPERT diffractometer (Bragg-Brentano configuration) with XCelerator RTMS detector and a Bruker D8 Advance X-ray diffractometer all using Cu Kα radiation (λ = 1.5406 Ǻ). In order to avoid the exposure of the sample to the air a particular sample holder equipped with an airtight Kapton foil was used. A part of the XRD characterization was performed outside of Helmholtz-ZentrumGeesthacht at the JRC’s Institute for Energy in Petten (Netherland). The measurements performed at JRC´s were carried out by Dr. Francesco Dolci. For these measurements the material was dispersed in high vacuum grease and deposited on a aluminum plate.

2.6 In situ synchrotron radiation powder X-ray diffraction

The phase evolution during the absorption/desorption process were investigated by in situ synchrotron radiation powder X-ray diffraction (SR-PXD). The measurements were performed in transmission mode at the beamline I711 at the Max II synchrotron in Lund. The radiation source was a 1.8 T multipole wiggler. The beam was focused vertically by a bendable mirror and horizontally by an asymmetrically cut Si (111) monochromator ^[35]. The range of available wavelength was 0.08-0.155 nm. The I711 beamline was equipped with a MAR 165 charge coupled device (CCD) plate detector, with a pattern recording time of 15-20 seconds. A special sample holder designed for in–situ monitoring of solid/gas reactions was utilized [33, 36, 37]. Hydrogen pressures up to 150 bar and temperatures up to 600 °C could be applied.

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2.7 Electron microscopy

The microstructure of the system 2NaBH4+MgH2 was investigated upon hydrogen desorption and absorption by means of electron microscopy. Several specimens were prepared at Helmholtz-Zentrum Geesthacht and FZK using Sievert´s type apparatus, HP-DSC and then characterized by transmission electron microscopy (TEM) and scanning electron microscopy (SEM). The TEM analyses were performed in the department of materials at the University of Oxford together with Dr. Christopher Nwakwuo, Dr. John Sykes and Dr. John Hutchison.

Material handling and sample preparation for TEM were done in a dedicated glove box under a purified argon atmosphere. A ground powder sample was mixed with n-hexane and a drop of the supernatant liquid placed on a lacey-carbon copper TEM grid. TEM and high resolution electron microscopy (HREM) investigation were carried out using the JEOL-JEM-3000F field emission gun (FEG) microscope at 300 kV with a point resolution of 0.16 nm. It is also equipped with a high angle annular dark field (HAADF) detector, Gatan imaging filter (GIF), Mega scan CCD camera and an Oxford Instrument energy dispersive X-ray (EDX) spectrometer.

The materials were analyzed by SEM at Helmholtz-Zentrum Geesthacht. SEM investigations were carried out using the ZEISS DSM 962 Scanning electron microscope. The material was spread onto a double sided adhesive, electrically conductive carbon sheet under a continuously purified argon atmosphere and then transferred in the electron microscope.

Although, particular attention in avoiding the oxygen contamination of the specimens was paid, this could not be ensured completely. This is true in particular for the SEM measurements where the samples had to be directly exposed to the atmospheric air for roughly thirty seconds in order to transfer them into the machine.

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2.8 Solid state nuclear magnetic resonance

Several specimens with different hydrogenation degree were prepared at Helmholtz- Zentrum Geesthacht and FZK using Sievert´s type apparatus, HP-DSC and then characterized by solid state Nuclear Magnetic Resonance (NMR) technique at the Servei de Ressonància Magnètica Nuclear (SeRMN) (Universitat Autònoma de Barcelona, 08193 Bellaterra, Spain).

The measurements were performed by Dr. Sebastiano Garroni.

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Figure 2.1: (a) ZrO2 rotors and (b) Bruker Avance 400 MHz spectrometer at theServei de Ressonància Magnètica Nuclear (SeRMN) Universitat Autònoma de Barcelona, Bellaterra, Spain.

Solid state NMR under Magic Angle Spinning conditions (MAS) has recently been demonstrated to be a powerful technique to study materials for hydrogen storage. Due to the fact that the ¹¹B chemical shift allows a fast identification of the boron containing species, this technique was successfully implemented for the study of metal borohydrides and alumino boranes [33, 38-40]. In this study, we used single pulse ¹¹B{¹H}NMR and ²³Na{¹H}NMR under MAS conditions in order to visualize the chemical state of boron and sodium during hydrogen desorption and absorption of the system 2NaBH4+MgH2. Nuclear Magnetic Resonance

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(NMR) spectra were measured using a Bruker Avance 400 MHz spectrometer (figure 1.1 b) with a wide bore 9.4 T magnet and employing a boron-free Bruker 4 mm CPMAS probe. The spectral frequencies were 128.33 MHz for ¹¹B nucleus and 105.85 for the ²³Na nucleus, the NMR shifts are reported in parts per million (ppm) externally referenced to BF3·O(CH2CH3)2

and NaCl respectively. In the NMR analysis reported in this work the main peak spinning side bands are indicated with the sign *. The powder materials were packed into 4 mm ZrO2 rotors (figure 1 A) in an argon-filled glove box and were sealed with tight fitting Kel-F caps. Sample spinning was performed using dry nitrogen gas. The one dimensional (1D) ¹¹B MAS NMR spectra were acquired after a 2.7 μs single π/2 (corresponding to a radio field strength of 92.6 kHz) and with application of a strong ¹H decoupling by using the two-pulse phase modulation (TPPM) scheme ^[41]. The recovery delay was set to 10 seconds. Spectra were acquired at 20ºC temperature controlled by a BRUKER BCU unit.

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3 Results

The absorption and desorption processes of the system NaBH4-MgH2 were studied in detail by means of volumetric measurements, calorimetric techniques, in and ex situ XRD analysis, electron microscopy (TEM and SEM) and MAS NMR characterization of selected samples.

In this section the hydrogen absorption process will be characterized and the effects of the applied hydrogen pressure and of the NaH/MgB2 ratio will be investigated in detail. Then, in second part the hydrogen desorption process of the as milled 2NaBH4+MgH2 will be analyzed. Regarding the desorption process the beneficial effect of a pretreatment of the as milled material by a heat-hydrogen process or by exposure to water saturated atmosphere will be described also.

3.1 The first hydrogen absorption

A preliminary hydrogen absorption test on the as milled 2NaH+MgB2 was performed under 50 bar of hydrogen and at a final temperature of 400 °C. These conditions were chosen arbitrarily in order to ensure a fast hydrogen absorption reaction.

3.1.1 Volumetric analysis

The hydrogen absorption kinetics of the as milled 2NaH+MgB2 is shown in figure 3.1.

The measurement was performed at 50 bar of hydrogen pressure, heating the material from RT to 400 °C with a heating rate of 3 °C/min, and subsequently keeping it under isothermal conditions at 400 °C for several hours. The hydrogen absorption starts at roughly 250 °C and in two separate steps of about 0.6 and 3.2 wt.% reaches a total amount of absorbed hydrogen equal to 3.8 wt.%. During the isothermal period at 400 °C the hydrogen uptake apparently stops after ~ 1h without reaching the expected theoretical gravimetric hydrogen storage capacity (7.8 wt.%). Aiming to understand the reasons of the hydrogen uptake stop before

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achieving the theoretical storage capacity, the absorption products (roughly 120 mg) were investigated by XRD analysis (figure 3.2). Among the absorption products it is possible to observe the presence of NaBH4 (43.82 wt.% (±4)), NaMgH3 (19.14 wt.% (±2)) and free Mg (5.88 wt.% (±1)) in addition to the starting reactants NaH (22.07 wt.% (±2)) and MgB2 (5.10 wt.% (±1)) and a small amount of NaOH (3.54 wt.% (±1)). Surprisingly, NaMgH3 and free Mg are present instead of the expected MgH2. In fact, at 50 bar of hydrogen and 400 °C MgH2

is thermodynamically more stable than Mg. The presence of un-reacted material together with NaMgH3 and free Mg clearly justify the not achieved theoretical hydrogen storage capacity (7.84 wt.%). In addition, the material after absorption appears to “wets” the inner walls of the sample older. For this reason we assume that one or more absorption reaction steps occur via molten state.

Figure 3.1: Absorption kinetic of as milled 2NaH+MgB2 measured in a Sievert’s -Type apparatus. The sample was heated at 50 bar of hydrogen pressure from RT to 400 °C (heating rate of 3 °C/min) and then kept under isothermal conditions for several hours.

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Figure 3.2: XRD pattern of 2NaH+MgB2 after absorption under 50 bar of hydrogen pressure at 400 °C (wavelength = 0.154184 nm).

The XRD analyses shown in figure 3.2 describe the average composition of hydrogenated material. However, morphological differences between the material at the topmost and bottom part of the sample holder suggest a possible different material distribution within the same specimen. In fact, the material at the topmost part of the sample holder is dense whereas the material at the bottom of the sample holder is powdery. In order to investigate the material distribution a second batch of 2NaH+MgB2 was hydrogenated at the same conditions as used for the previous absorption. The material obtained was then divided in several parts. The portions of material at the topmost and bottom of the sample holder (roughly 5 mg each) were analyzed by XRD technique (figure 3.3). The weight fraction of all involved phases was calculated by Rietveld’s method and reported in table 3.1. Clearly, the compositions of the two materials are strongly different. The material at the topmost part of the sample holder (figure 3.3 A) includes a mixture of NaBH4 (19.77 wt.% (±2)), MgB2 (36.31 wt.% (±4)), NaMgH3 (10.27 wt.% (±1)), Mg (6.12 wt.% (±1)), NaH (19.81 wt.% (±2)), Na (7.31 wt.%

(±1)) and NaOH (3.28 wt.% (±1)). Differently, the material at the bottom of the sample holder (figure 3.3 B) contains NaBH4 (57.55 wt.% (±5)), MgB2 (22.14 wt.% (±2)), NaMgH3 (17.11 wt.% (±2)) and Mg (3.18 wt.% (±1)). These results clearly show how the distribution of the

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final absorption products results highly inhomogeneous although the composition of the starting material was homogeneous. Interestingly, the material in the upper part of the sample holder appears to be richer of the starting reactants whereas at the bottom of the sample holder the presence of the absorption products is predominant. It must be noticed that although Na is visible among the phases present at the top of the sample holder, its presence in the diffraction pattern of figure 3.2 was not detected. This is due to the fact that whereas the amount of Na present in the portion of sample analyzed (roughly 5 mg) in figure 3.3 A is significant, its amount is negligible when the all sample is considered (roughly 120 mg).

Figure 3.3: XRD pattern of 2NaH+MgB2 after absorption under 50 bar of hydrogen pressure at 400 °C (wavelength = 0.154184 nm): A) material at the top of the sample holder, B) material at the bottom of the sample holder.

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Phase Material at the top of the sample holder

Material at the bottom of the sample holder

NaBH4 19.77 wt.% (±2) 57.55 wt.% (±5)

MgB2 36.31 wt.% (±4) 22.14 wt.% (±2)

NaMgH3 10.27 wt.% (±1) 17.11 wt.% (±2)

Mg 6.12 wt.% (±1) 3.18 wt.% (±1)

NaH 19.81 wt.% (±2) 0 wt.%

NaOH 3.28 wt.% (±1) 0 wt.%

Na 7.31 wt.% (±1) 0 wt.%

Table 3.1: Phases weight fraction (wt.%).

3.1.2 Thermal analysis

In order to visualize the sequence of events taking place during the heating of the 2NaH+MgB2 in hydrogen pressure a HP-DSC analysis was performed. Figure 3.4 shows the HP-DSC trace recorded at 50 bar of hydrogen pressure, measured from room temperature to 400 °C and then cooled to room temperature (constant heating/cooling rate 5 °C/min). This measurement shows during heating the presence of three main peaks: two exothermic events with respective onsets at 270 °C and 353 °C and one sharp endothermic peak at 330 °C.

During cooling, two exothermic peaks with onset temperature of 367 °C and 316 °C are observed.

Aimed at investigating the whole absorption process, a second HP-DSC analysis was performed (figure 3.5). The material was heated up to 400 °C under 50 bar of hydrogen pressure and then kept at 400 °C for 90 minutes before being cooled down to room temperature (heating/cooling rate of 5 °C/min.). The heating period of the HP-DSC curve shown in figure 3.5 clearly traces out the HP-DSC analysis previously reported in figure 3.4.

During the isothermal period at 400 °C no significant further events were observed. However,

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the cooling segment (figure 3.5) differently from the trace of figure 3.4 shows the presence of only a single exothermic signal with onset at 367 °C.

Figure 3.4: HP-DSC trace of the 2NaH+MgB2 absorption reactions, measured at 50 bar of hydrogen pressure from RT to 400 °C and subsequently cooled (5 °C/min heating/cooling rate).

Figure 3.5: HP-DSC trace of the 2NaH+MgB2 absorption reactions, measured at 50 bar of hydrogen pressure from RT to 400 °C and then kept under isothermal condition at 400 °C for 90 minutes and subsequently cooled (5 °C/min heating/cooling rate).

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3.1.3 In situ SR-PXD characterization

With the aim to clarify the undergoing hydrogenation mechanisms in situ SR-PXD patterns were measured. Figure 3.6 shows the measurement carried out at 50 bar of hydrogen pressure, in scanning temperature from RT to 400 °C and then cooling to 240 °C with a heating/cooling rate of 5 °C/min. The phases in the starting material are NaH and MgB2. Upon heating, due to thermal expansion all peaks shift continuously towards lower 2Ө angles.

At roughly 280 °C the formation of an unknown crystalline phase with major reflection at 14.36, 16.59, 19.54, 23.54 and 27.792Ө angle (wavelength = 0.1072 nm) is observed. This phase is found to be stable up to 325 °C, and then its diffraction peaks disappear. Moreover at roughly 330 °C the formation of NaMgH3 starts. This is accompanied by a significant decrease of the NaH diffracted intensity. Formation of NaMgH3 continues up to a temperature of 350 °C. NaMgH3 formation and the disappearance of NaH are followed by the formation of an amorphous background at 19.50 2Ө angle. After forming, this amorphous background remains almost constant until the temperature reaches 400 °C. At 380 °C reflections of crystalline NaBH4 appear and continuously grow until the final temperature 400 °C is reached.

The cooling period is characterized by the progressive intensity rising of the NaH reflections plus two more events, which take place at 370 and 320 °C, respectively. At roughly 370 °C the intensity of NaBH4 and NaH peaks quickly rise and later at about 320 °C, the peaks related to the unknown phase observed during the heating period reappear. Simultaneously with these two events which are described above, the amorphous background disappears completely. In a second experiment the starting material 2NaH+MgB2 was heated to 400 °C in 50 bar H2 and then kept at this temperature. The temperature and pressure at which the measurement was performed were chosen in order to simulates the absorption process carried out in the Sievert´s type apparatus (figure 3.7, wavelength = 0.109719 nm). The sequence of events taking place during the heating period, exactly trace out those described for figure 3.6.

The isothermal period at 400 °C is characterized by the complete disappearing of the NaH peaks and the growth of NaBH4 phase, which takes place within the first 30 minutes of the isothermal period. It must be noticed, that NaBH4 formation is followed by the simultaneous appearance of free Mg.

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Figure 3.6: Series of SR-PXD patterns of the 2NaH+MgB2 system heated under 50 bar hydrogen pressure from RT to 400 °C and cooled to 240 °C (5 °C/min, wavelength = 0.1072 nm). The measurement was obtained at the beamline I711 at the Max II synchrotron in Lund.

Figure 3.7: Series of SR-PXD patterns of the 2NaH+MgB2 system heated under 50 bar hydrogen pressure from RT to 400 °C and then kept under isothermal condition (5 °C/min, wavelength = 0.109719 nm. The measurement was obtained at the beamline I711 at the Max II synchrotron in Lund.

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3.1.4 TEM investigation

The microstructural evolution of the material during the absorption process was characterized by means of TEM. Figure 3.8 shows the high resolution TEM image (picture a) and the XRD pattern (picture b) of the as milled 2NaH+MgB2. In the diffraction pattern of figure 3.8 b NaH and MgB2 are observed as the component phases of the system. Hence, neither significant decomposition nor contamination (e.g. oxygen, iron) of the starting materials took place during the milling period. These two phases are clearly visible also in the high resolution image of figure 3.8 a were based on an inter-planar distance of 0.283 and 0.2127 nm respectively the family of plan (111) for NaH and the (101) for MgB2 are identified. On the left hand side of figure 3.8 a the presence of an amorphous region is visible.

This region is most probably generated by the destructive interaction between the electron beam and the sample. Analyzing the diffraction pattern (figure 3.8 b) by the Rietveld´s method using the program MAUD an average crystallite size of 63 and 38 nm respectively for NaH and for MgB2 was calculated.

The high-resolution TEM image of the material heated at 50 bar of hydrogen pressure up to 310 °C and subsequently cooled to room temperature is shown in figure 3.9 a. The material appears to be constituted in the majority by un-reacted NaH and MgB2 plus a small fraction of an unidentified phase, which with all probability is the unknown phase observed in the SR- PXD analysis of figure 3.6 and 3.7. In figure 3.9 b the high resolution TEM analysis of material hydrogenated at 400 °C under 50 bar of hydrogen pressure is reported.

In this analysis, NaH, MgB2, NaBH4 and NaMgH3 are visible. In particular the (111) planes of NaH, the (101) of MgB2, the (200) of NaBH4 (inter-planar distance equal to 0.307 nm) and (111) for NaMgH3 (inter-planar distance equal to 0.272 nm) are recognizable. Differently from the SR-PXD analysis of figure 3.7 the free Mg is not observed. The phases in figure 3.9 b are distributed in the following way: the top side and the bottom side are occupied respectively by un-reacted NaH and MgB2. On the right hand side NaMgH3 first and the NaBH4 after are visible. The formation of NaMgH3 and NaBH4 seems to take place preferentially around MgB2. In fact a high overlapping of the lattice planes of MgB2 with the

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lattice planes of NaMgH3 and NaBH4 is noticed. No overlapping between the lattice planes of NaH and NaMgH3 or NaBH4 can be observed.

(a) High resolution TEM image (b) XRD pattern

Figure 3.8: (a) High resolution TEM image and (b) XRD patter of the as milled 2NaH+MgB2. The measurement was obtained at the University of Oxford using the JEOL- JEM-3000F field emission gun (FEG) microscope.

(a) (b)

Figure 3.9: TEM image of 2NaH+MgB2 hydrogenated under 50 bar of hydrogen pressure at 310 °C (a) and completely hydrogenated at 400 °C (b). The measurement was obtained at the University of Oxford using the JEOL-JEM-3000F field emission gun (FEG) microscope.

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3.1.5 MAS NMR analysis

Due to the possible formation of amorphous compounds, the employment of the MAS NMR technique has been necessary to further elucidate the 2NaH+MgB2 hydrogen absorption reaction. Several specimens with different hydrogenation degrees were prepared at a hydrogen pressure of 50 bar in a HP-DSC Netzsch DSC 204 HP Phoenix and then characterized. The single pulse ¹¹B{¹H} and ²³Na {¹H}NMR analysis are shown in figure 3.10 and figure 3.11 respectively. The ¹¹B{¹H} spectra collected for the as milled material (figure 3.10 A) shows a peak at 96.51 ppm, due to the presence of MgB2 and although small, a further signal is visible at -42.00 ppm. The presence of this last signal (at -42.00 ppm), suggests a partial formation of NaBH4 already during milling, however it does not find confirmation in the ²³Na{¹H} MAS NMR analysis of figure 3.11 A. In fact, despite the signal observed at -42.00 ppm in figure 3.10 A, the ²³Na{¹H} spectrum of the as milled 2NaH+MgB2 (figure 3.11 A) shows only the signal related to NaH (10.84 ppm). Therefore, the signal at -42 ppm is not due to the formation of NaBH4. The investigation of the phase generating the signal at -42.00 ppm (figure 3.11 A) is still in progress.

A first hydrogen charged specimen was prepared by heating the as milled material from room temperature up to a final temperature of 300 °C (heating rate 5 °C/min) under a pressure of 50 bar of hydrogen, and subsequently cooling it down to room temperature. According to the HP-DSC analysis of figure 3.4 and the SR-PXD data shown in figure 3.6, this sample is expected to contain only the observed unknown crystalline phase together with the starting reactants. The presence of this additional phase among the starting reactants is confirmed by both ¹¹B{¹H} and ²³Na NMR analysis. In the ¹¹B{¹H} spectra (figure 3.10 B) an intensity increment of the already present peak at -42.00 ppm is clearly visible in addition to the signal at 96.51 ppm of the B- atoms contained in the MgB2. Moreover with respect to the as milled material (figure 3.11 A), the ²³Na{¹H} spectra (figure 3.11 B) shows the presence of two more signals at -11.71 and at -15.85 ppm. These two new peaks hint to the presence of the unknown crystalline phase. The last sample was prepared by heating the as milled material from room

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temperature up to a final temperature of 400 °C (heating rate 5 °C/min) under 50 bar of hydrogen, and finally keeping it at 400 °C for 2 hours. For this material the ¹¹B{¹H} NMR spectra (figure 3.10 C) clearly shows at -42.29 ppm a peak relative to the NaBH4 presence, plus an additional signal at -15.97 ppm. Analyzing the region of positive shifts, in addition to the peak of the remaining MgB2 at 96.51 ppm, three more signals are visible. A broad signal at 3.09 ppm (overlapped with the spinning sideband of MgB2), and two sharp peaks at 6 and 18 ppm. Although, most likely the peak observed at 3.09 ppm is due to the formation of amorphous boron, due to the low signal proportion (less than 1% of total boron signal) an assignment for the three remaining B-containing species (-15.97, 6.16 and 18.20 ppm) is rather difficult. For this reason, a direct spectral sensitivity comparison between, equally recorded, proton decoupled boron experiment ¹¹B{¹H} and proton coupled boron experiment

11B was performed. This analysis allows distinguishing between species which contain boron atoms strongly coupled to protons (directly bonded) and those which are not. Several strategies are known for performing hetero nuclear decoupling. Herein, for CPD (Composite Pulse Decoupling) we employed TPPM technique (Two Pulse Phase Modulation). Figure 3.12 shows the ¹¹B MAS NMR spectra of as milled 2NaH+MgB2 hydrogenated at 50 bar and 400 °C with proton CDP (blue line) and without proton CDP (black line). Clearly the proton decoupling leads to an enhancement of the signal relative to the [BH4]^- anion at -42.29 ppm and of its spinning sidebands at 144.31, 61.62 and -135.33 ppm. Different behaviour is observed for the signals at 18.20, 6.16, 3.09 and -15.97 ppm. For these signals the application of the proton CDP does not influence the intensity of the signals. This behaviour is due to the fact that these species contain B-atoms which are not strongly coupled to hydrogen atoms.

This suggests the formation of species without B-H bonds. Spectrum C in figure 3.11 shows the ²³Na{¹H}NMR analysis of the material hydrogenated at 400 °C and 50 bar of hydrogen. It is possible to observe the peak of remaining NaH at 10.84 ppm and the signal of the Na contained in NaBH4 at -15.85 ppm. A further broad signal is observed at 2.32 ppm. Most likely this signal is related with the peaks observed at 18.20, 6.16 and -15.97 ppm in the

11B{¹H} NMR analysis of figure 3.10 spectrum C.

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Figure 3.10: A) ¹¹B{¹H} MAS (12 kHz) single pulse NMR spectrum of: as milled 2NaH+MgB2. B) as milled 2NaH+MgB2 heated from room temperature up to a final temperature of 300 °C under a pressure of 50 bar of hydrogen. C) as milled 2NaH+MgB2

heated from room temperature up to a final temperature of 400 °C and then kept at 400 °C for 2 hours under a pressure of 50 bar of hydrogen.

Figure 3.11: A) ²³Na{¹H} MAS (12 kHz) single pulse NMR spectrum of: as milled 2NaH+MgB2. B) as milled 2NaH+MgB2 heated from room temperature up to a final temperature of 300 °C under a pressure of 50 bar of hydrogen. C) as milled 2NaH+MgB2

heated from room temperature up to a final temperature of 400 °C and then kept at 400 °C for 2 hours under a pressure of 50 bar of hydrogen.

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Figure 3.12: ¹¹B{¹H} MAS (12 kHz) single pulse NMR spectrum of as milled 2NaH+MgB2

heated from room temperature up to a final temperature of 400 °C and then kept at 400 °C for 2 hours under a pressure of 50 bar of hydrogen (black line) and ¹H decoupled ¹¹B MAS (12 KHz) single pulse NMR spectrum of as milled 2NaH+MgB2 heated from room temperature up to a final temperature of 400 °C and then kept at 400 °C for 2 hours under a pressure of 50 bar of hydrogen

3.2 Effect of the hydrogen pressure on the absorption reaction

In the previous section the 2NaH+MgB2 hydrogen absorption at a pressure of 50 bar was investigated. It was observed that at 50 bar H2 pressure NaBH4 is not formed directly.

Instead, first an unknown crystalline phase is formed, followed upon further heating by the formation of NaMgH3 and additional unidentified phases. In this section the effect of the applied hydrogen pressure on the hydrogenation process of the system 2NaH+MgB2 is described. In particular the hydrogen absorption processes at 25 and 5 bar were studied by means of volumetric measurements, HP-DSC technique, in situ SR-PXD, MAS NMR and then compared with the hydrogen absorption process carried out at 50 bar.

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3.2.1 Volumetric analysis

Figure 3.13 shows the hydrogen absorption kinetics for the system 2NaH+MgB2

measured under 50, 25 and 5 bar of hydrogen pressure (respectively curve A,B and C) heating the material from RT to 400 °C (heating rate 3 °C/min) and subsequently keeping it under isothermal conditions at 400 °C. The volumetric measurement performed at 50 bar of hydrogen pressure previously discussed (figure 3.1) was added to figure 3.13 for comparison purposes.

The absorption curve of as milled 2NaH+MgB2 measured at 25 bar of hydrogen pressure (Figure 3.13 B) similarly to curve A (figure 3.13) shows a multi-step hydrogen absorption kinetic. The first step starts at around 300 °C. The amount of hydrogen stored in this stage is equal to 0.7 wt.%. Upon further heating a second hydrogen absorption step starts roughly at 350 °C and continues at 400 °C. The measurement was stopped after 63 hours, when the total amount of hydrogen charged in the system reached 6.2 wt.%. Considering that the measurement was performed at a hydrogen pressure much lower than the previous experiment, the amount of hydrogen stored in the system is rather surprising. A further absorption measurement was performed at a pressure of 5 bar only (Figure 3.13 C). In this case the hydrogen uptake starts at roughly 330 °C, and then continues at 400 °C. After 45 hours an amount of hydrogen equal to 3.8 wt.% was stored in the system. Differently from the first two absorption measurements, the experiment performed at 5 bar hydrogen pressure (figure 3.13 C) does not show a first marked absorption step. The volumetric analysis shows a clear dependence of the absorption kinetics on the hydrogen pressure at which the measurements are performed. However, the causes for this are not yet understood. Therefore, to better understand the effect of the hydrogen pressure on the absorption reaction, XRD measurements of the absorbed materials were performed. The diffraction patterns of the material after hydrogen absorption at 400 °C and 50, 25 and 5 bar of hydrogen pressure respectively (pattern A, B and C)are shown in figure 3.14. The diffraction pattern of the material obtained after hydrogen absorption at 25 bar of hydrogen pressure (figure 3.14 B) shows exactly the same reflections observed for the material synthesized at a pressure of 50 bar (figure 3.14 A).

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(a) (b)

Figure 3.13: Absorption kinetics of as milled 2NaH+MgB2 measured in a Sievert’s -Type apparatus. The samples were heated under 50, 25 and 5 bar hydrogen pressure from RT to 400 °C (curve A,B and C) using a heating rate of 3 °C/min. (a) complete measurements, (b) firsts three hours of absorption.

Figure 3.14: XRD patterns of 2NaH+MgB2 after absorption at 50, 25 and 5 bar of hydrogen pressure and 400 °C (respective patterns A, B and C, wavelength = 0.154184 nm).

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Significant differences can be noticed in the respective diffraction phase intensities. In fact, the ratios of peak intensities of NaH and NaMgH3 reflections to those of NaBH4, are inferior for the material obtained at a pressure of 25 bar than for the material prepared at 50 bar. For example, the intensity ratio of the peaks NaH (111) at 31.60 2Ө angle and NaMgH3 (200) at 32.81 2Ө angle to the NaBH4 peak (220) at 41.26 2Ө angle, changes respectively from 1.49 and 1.31 for the material charged at 50 bar to 0.42 and 1.17 for the material charged at 25 bar H2 pressure.

This indicates a more marked formation of NaBH4 for the measurement performed at 25 bar hydrogen pressure. The application of just 5 bar leads to quite different results. Pattern C (figure 3.14) shows the formation of only NaBH4, and the presence of free Mg together with the starting reactants. NaMgH3 could not be detected among the final absorption products.

Although the simultaneous presence of unreacted materials, together with NaMgH3 and free Mg, well justify the not achieved theoretical capacity, the reason of the formation of NaMgH3

and free Mg is an issue which will be addressed later in this work. As for the material absorbed at a pressure of 50 bar of hydrogen the materials hydrogenated at 25 and 5 bar appear to “wet” the inner walls of the sample older.

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3.2.2 Thermal analysis

Figure 3.15: HP-DSC traces of the 2NaH+MgB2 absorption reactions, measured at 50 (pattern A), 25 (pattern B), 5 bar of hydrogen pressure (pattern C) from RT to 400 °C and subsequently cooled (5 °C/min heating/cooling rate).

In order to visualize the sequence of events taking place during absorption at different hydrogen pressures, HP-DSC analyses were performed. Figure 3.15 shows the HP-DSC traces recorded at 50 (A), 25 (B), 5 bar (C) of hydrogen pressure measured from room temperature to 400 °C and then cooled to room temperature (constant heating/cooling rate 5 °C/min).

Trace A (figure 3.15) was previously discussed in section 3.1.2 and is here reported for comparison purpose. Curve B (figure 3.15), similarly to the measurement performed under 50 bar hydrogen pressure (Figure 3.15 A), shows a main exothermic peak at 284 °C, and a small endothermic signal at 330 °C. Due to the lowered pressure (25 instead of 50 H2 bar) the onset of the absorption process is shifted to higher temperatures. Equally to the HP-DSC measurement carried out at 50 bar (figure 3.15 A) the cooling period is characterized by the presence of two strong exothermic signals with onsets at 367 °C and 316 °C. The HP-DSC trace measured at 5 bar hydrogen pressure (figure 3.15 C), shows, upon heating, as in case of the curve measured at 25 bar two exothermic signals. The onset temperature is now shifted to 290 °C and 320 °C. In contrast to the measurements carried out at 50 and 25 bar the cooling period is characterized by the presence of one single exothermic event with an onset

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temperature of 367 °C. The second exothermic peak at 316 °C is missing. These findings confirm that the hydrogen absorption process for the 2NaH+MgB2 system is a multi-steps reaction sensible to the hydrogen pressure at which the measurements is performed.

Moreover, the presence of sharp endothermic peaks during heating and sharp exothermic peaks during the cooling-process strengthens our hypothesis that the absorption reaction might occur via molten phase.

Although, the amount of hydrogen stored in the system at 25 bar of hydrogen pressure is sensibly different from that stored at 50 bar, based on the HP-DSC trace (figure 3.15 B) and the XRD analysis (figure 3.14 B) we assume for the two of them a common reaction path.

3.2.3 In situ SR-PXD characterization

In order to investigate the hydrogenation reaction mechanisms of the composite system 2NaH+MgB2 at 5 bar H2 pressure an in situ SR-PXD analysis was carried out. The material was heated up from room temperature to 400 °C and then held isothermally at this temperature for 3 hours. Finally, the material was cooled down to 60 °C (figure 3.16, heating/cooling rate 5 °C/min). The phases in the starting material are NaH and MgB2. Apparently, no reaction can be observed below 320 °C. At this temperature the intensity of NaH and MgB2 peaks start to decrease. Simultaneously an amorphous background starts to form at about 19.50 2Ө angle. Within the first 10 minutes of the isothermal period at 400 °C the complete disappearance of NaH and the formation of free Mg are observed. No significant changes are observed during the remaining part of the isothermal period. During the cooling, at roughly 370 °C the instantaneous formation of NaBH4 and partial reformation of NaH take place. These two events are followed by the vanishing of the amorphous background (figure 3.16). No further changes occurred during cooling until the measurement was stopped at 60

°C.

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Figure 3.16: Series of SR-PXD patterns of the 2NaH+MgB2 system heated under 5 bar hydrogen pressure from RT to 400 °C and kept under isothermal condition for 3 hours before to cool it down to 60 °C (5 °C/min, wavelength = 0.1072 nm). The measurement was obtained at the beamline I711 at the Max II synchrotron in Lund.

3.2.4 MAS NMR analysis

The hydrogen absorption process performed under a pressure of 5 bar was also characterized by MAS NMR technique. The material was prepared heating it up to 400 °C under a hydrogen pressure of 5 bar and then holding it under isothermal and isobaric conditions for 40 hours. The ²³Na{¹H} and the ¹¹B{¹H}NMR spectra of the as milled material are here presented again for comparison purpose. The ²³Na{¹H}NMR analysis of the hydrogenated material (figure 3.17 B) shows at 10.88 ppm the signal of the remaining NaH and at -15.65 ppm that due to the NaBH4 formation. The¹¹B{¹H} NMR spectra (figure 3.18 B) shows a peak at 96.61 ppm, due to the presence of not reacted MgB2 and a strong signal at - 42.76 ppm, which clearly confirms the formation of NaBH4.

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Figure 3.17: A) ²³Na{¹H} MAS (12 kHz) single pulse NMR spectrum of: as milled 2NaH+MgB2. B) as milled 2NaH+MgB2 heated from room temperature up to a final temperature of 400 °C under a pressure of 5 and then kept at 400 °C for 40 hours under a pressure of 50 bar of hydrogen.

Figure 3.18: A) ¹¹B{¹H} MAS (12 kHz) single pulse NMR spectrum of: as milled 2NaH+MgB2. B) as milled 2NaH+MgB2 heated from room temperature up to a final temperature of 400 °C under a pressure of 5 and then kept at 400 °C for 40 hours under a pressure of 50 bar of hydrogen.

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3.3 Effect of NaH/MgB

2

ratio on the absorption kinetics

In this chapter the possible effect which a different ratio between reactants could trigger on the absorption properties of the NaH-MgB2 system is investigated. Regarding this topic, two works were recently published by Price et al. ^[34] and Garroni et al. . In their works, they investigated the effect of the reactants ratio on the sorption properties of the systems LiBH4-MgH2 and NaBH4-MgH2. However, no clear understanding of the physical effects which lie behind the changed sorption properties was obtained. In order to achieve such a fundamental knowledge the hydrogen absorption behavior of the compositions 1.5Na/MgB2, NaH/MgB2 and 0.5NaH/MgB2 were studied by means of volumetric, HP-DSC and in situ – ex situ XRD techniques.

3.3.1 Volumetric analysis

The measurements reported here were done at the JRC’S Institute for Energy in Petten. The absorption kinetics were measured at 50 bar of hydrogen pressure, heating the material from room temperature up to 400 °C (heating rate 3 °C/min) and then keeping the material under constant temperature at 400 °C. Figure 3.19 shows the absorption measurements performed for the compositions 2NaH/MgB2 (A), 1.5Na/MgB2 (B), NaH/MgB2 (C) and 0.5NaH/MgB2

(D). Differently from the measurement reported in figure 3.1 curve A, the absorption reaction reported in figure 3.19 A shows a three steps absorption kinetic. As in case of figure 3.1 curve A the first absorption step starts at roughly 300 °C and continues until an amount of approximately 0.6 wt.% hydrogen is stored in the system. Then the second reaction step starts and continues to absorb hydrogen for additional 14 hours. Then, after absorption of 5.3 wt.%

hydrogen a last absorption step occurs charging further 0.45 wt.% of hydrogen. The final amount of absorbed hydrogen was 5.75 wt.%. The observed absorption reaction for the system 1.5Na/MgB2 (figure 3.19 B) traces out the reaction kinetic observed for the system 2NaH/MgB2 (figure 3.19 A). However, this time the previously observed third absorption step already starts after 9 hours only at a hydrogen content of 4.4 wt.%. The measurement was stopped after 22 hours and the final amount of hydrogen stored in the system was 4.86 wt.%.

The observed absorption reaction for the system NaH/MgB2 (figure 3.19 C) is divided in two

Hydrogen sorption properties of the composite system 2NaBH

Hydrogen sorption properties of the composite system 2NaBH 4 +MgH 2

Hydrogen sorption properties of the composite system 2NaBH

+MgH

Claudio Pistidda

Wasserstoffspeichereigenschaften des Kompositen-Systems 2NaBH

+MgH

Claudio Pistidda

Contents

1 Introduction

1.1 Complex hydrides for hydrogen storage

1.2 Scope of the work

2 Experimental section

2.1 Materials

2.2 Sample preparation

2.3 Kinetic characterization

2.4 Thermal analysis

2.5 Ex situ X-ray diffraction

2.6 In situ synchrotron radiation powder X-ray diffraction

2.7 Electron microscopy

2.8 Solid state nuclear magnetic resonance

3 Results

3.1 The first hydrogen absorption

3.1.1 Volumetric analysis

3.1.2 Thermal analysis

3.1.3 In situ SR-PXD characterization

3.1.4 TEM investigation

3.1.5 MAS NMR analysis

3.2 Effect of the hydrogen pressure on the absorption reaction

3.2.1 Volumetric analysis

3.2.2 Thermal analysis

3.2.3 In situ SR-PXD characterization

3.2.4 MAS NMR analysis

3.3 Effect of NaH/MgB

ratio on the absorption kinetics

3.3.1 Volumetric analysis

Hydrogen sorption properties of the composite system 2NaBH ₄ +MgH ₂